Tertiary aliphatic methyl amines are very useful compounds, finding applications in such various areas as corrosion inhibitors, fuel oil additives, bactericides, fungicides, pigment grinding, ore flotation and as intermediates in the preparation of a wide range of other chemicals, such as quaternary ammonium compounds. Although this versatility has been long known, full utilization of tertiary aliphatic methyl amines has been restricted because of their traditionally high price. Although this disadvantage has been due in part to the cost of the raw materials from which tertiary aliphatic methyl amines are manufactured, the principal cause for their expense has been the lack of commercially acceptable processes for their preparation. There has additionally been no viable means of producing tertiary aliphatic methyl amines by a continuous process which gives good selectivity to the tertiary amines over the co-produced and less valuable secondary and primary by-products. Prior continuous processes also generate catalyst fines which cause reactor plugging problems. To avoid this, prior methods are conducted in batch reactors rather than in continuous systems.
Other known processes for the production of tertiary aliphatic methyl amines includes that described in U.S. Pat. No. 3,136,819. This patent describes a batch process for preparing tertiary aliphatic methyl amines where formaldehyde is progressively added to an amine of the formulae R.sub.1 NH.sub.2, (R.sub.1).sub.2 NH, R.sub.1 R.sub.2 NH and R.sub.1 NH(CH.sub.2).sub.3 NH.sub.2, where R.sub.1 and R.sub.2 represent aliphatic hydrocarbon radicals having from 8 to 22 carbon atoms, maintaining the reactants in the liquid phase under hydrogen pressure at reaction temperature between about 50.degree. and 175.degree. C. and 100-250 psig pressure. The amine contains a hydrogenation catalyst, such as a Raney nickel catalyst. The improvement consisting of employing as an additional catalyst from about 0.5 to about 3 wt. %, based on the amine, of an acid selected from the group consisting of short-chain aliphatic monobasic carboxy, hydroxy monobasic carboxy and dibasic carboxy acids, and benzoic acid. The suitable monobasic acids and hydroxy monobasic acids have at least two carbon atoms, and the dibasic acids have at least three carbon atoms.
U.S. Pat. No. 4,248,801 describes a batch process for making tertiary monomethylamines having long chain alkyl groups from unsaturated aliphatic nitriles under low pressure at high yield by three steps. The first step involves reducing the nitriles with hydrogen in the presence of a nickel hydrogenation catalyst at 200.degree. through 230.degree. C. and under a hydrogen pressure of 0 through 10 kg/cm.sup.2 G, while the formed ammonia is removed. The second step concerns reacting the resulting amines from the first step with a hydroxymethylation agent in the presence of the nickel catalyst at 150.degree. through 180.degree. C., while hydrogen is passed through the reaction zone under 0.3 through 7 kg/cm.sup.2 G and the formed water is removed. The last step reduces the resultant tertiary amines with hydrogen in the presence of the nickel catalyst at 175.degree. to 210.degree. C. and under a hydrogen pressure of 5 through kg/cm.sup.2 G. The patent emphasizes that the third step should not precede the second step.
Of somewhat lesser importance is the three step process described by H. Abe, et al. in "Research on the Synthesis of N,N-di(long-chain alkyl)-methylamines," Yukagaku, Vol. 38, No. 1, (1989) pp. 94-98 (Chemical Abstracts 111:96617r). The article describes the production of tertiary methyl amines from the corresponding alcohol by dehydrogenation, amine addition and hydrogenolysis. The addition of amine (methylamine) proceeds without a catalyst, but copper is a suitable dehydrogenation catalyst and nickel is used as catalyst for the hydrogenolysis. The catalysts may be combined as a copperr-nickel catalyst and used for both steps.
There remains a need for a continuous single-step process for producing tertiary alkyl methyl amines from dialkyl secondary amines simply, and in high selectivities. The batch processes of the patents described above are not believed suitable for adaptation to continuous processes. As noted, the physical stability of prior and current commercial catalysts has been too short and would be expected to cause reactor plugging problems with fines if used to produce commercial quantities of product. Additionally, it is preferred that only one catalyst be used to help reduce process complexity and costs.